1. Field of the Invention
The present invention relates generally to an insulation deterioration monitoring apparatus and more specifically to an apparatus for monitoring the insulation deterioration of high voltage coils provided for high voltage machines such as induction motors, generators, etc.
2. Description of the Prior Art
The background of the present invention will be explained with respect to its application to coils engaged with an iron core of a high voltage rotating machine. Within an insulating layer formed on a high voltage coil, gaps such as voids or exfoliation readily occur for some reason or other in the manufacturing processes or due to cooling and heating cycles during operation (machines are heated in operation and cooled at rest). These gaps will cause partial electric discharge whenever a high electric field is applied. Once the partial discharge occurs, the coil insulating layer is eroded gradually, while reducing the dielectric strength, resulting in a serious dielectric breakdown accident finally.
In order to solve the above problems, although it may be possible to reduce the partial electric discharge generated within the insulation layer by eliminating the gaps formed in the insulation layer, it is usually very difficult to completely eliminate the gaps when considering various causes.
Therefore, the insulation characteristics are sufficiently inspected for each coil independently and further the insulation characteristics of the whole coil assembly are inspected after each coil has been engaged with the iron core, connected to each other, and treated by varnish coating process.
However, the above inspection is made for checking the initial manufacturing defective. Since the insulation deterioration according to a change with passage of time starts after operation, it is very important to inspect a change with passage of time in insulation characteristics after or during operation. Conventionally, there exists a problem in that a change with passage of time in insulation characteristics should be inspected after the machine has been stopped during operation. Therefore, the time interval between inspections inevitably be increased and it is impossible to grasp the insulation characteristics accurately at all times, thus resulting in an occurrence of an unexpected serious accident. Therefore, there exists a strong demand for establishing a technique for insulation diagnoses in the state where the machine is in operation.